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Background: Know: Recognize: Prerequisites - Interfaces (Networking), IP address, VLAN, ICMP echo(ping), UDP echo port(#7), HTTP TRACE method Loopback Definition: Loopback is a diagnostic test that involves a device reflecting back a signal that is received through a particular port across a link, allowing the initiator of the test to compare the original signal with the signal that was received back. ( Loopback services return what is received back to the originator.) Loopback can be used to test path to/from service. By comparing data that is sent and received over the looped connection, important conclusions can be made. For example, loopback tests can be used to verify that communications paths are indeed intact. Loopback tests are common in telecommunications. For instance, a connection can be setted up to a remote endpoint that is automatically "looped back" to where it originated--short-circuited. Loopback can also be used to measure centain quality-of-service parameters, such as delay. In addition, phone calls would be generated to test voice connections. Loopback tests can be carried out to test the functionality of ethernet, network and transport layers. Some Examples: Interfaces and IP: In TCP/IP, a loopback device is a virtual network interface implemented in software only and not connected to any hardware, but which is fully integrated into the computer system's internal network infrastructure. Any traffic that a computer program sends to the loopback interface is immediately received in the same interface. On Unic-like systems, the loopback interface usually has the device name lo of lo0 ( rather than eth0). Correspondingly, the Internet Protocol (IP) specifies a loopback network. The most commonly used IP address on the loopback device is 127.0.0.1 for IPv4. ICMP echo (ping): Ping is an ICMP echo request, and it can verify a network end-to-end. When a manager runs ping, the application generates and sends an ICMP Echo Request message to a specified destination. When an Echo Request arrives at a destination, ICMP software sends an ICMP Echo Peply back to the source. That is, ping can be used to test reachability of network devices and to measure network delay. Success means that routing is in place (in both directions) and routers along the path are forwarding packets correctly. In addition, pinging the loopback interface is a basic test of the functionality of the IP stack in the operating system. HTTP Trace: Trace is a HTTP request method used for debugging which echo's back input back to the user. UDP echo port 7: Used to trouble-shoot remote TCP/IP stacks. ( Telnet to remote echo port, then type all keystrokes will echo back if target stack is working thrugh application layer.）Using VLANs to create loopback: See slide "Ethernet loop-back". VLAN is a networking technology that allows network devices to behave as if they were connected to the same LAN, even though they might be physically located on separate LAN segments. VLAN for Ethernet loop-back: It can make a network manager test from site A if the link to a remote site B is working or not without leaving a host at each site. See also Corresponding TELE9752 lecture slides Loopback References A. Clemm: Network Management Fundamentals, Cisco Press, 2006 D. Comer: Automated Network Management Systems: Current and Future Capabilities, Pearson, 2007 Category:All